'Signature whistles' help dolphin calves identify their mums

London, July 24 : American researchers have discovered that female bottlenose dolphins whistle 10 times more often than usual after giving birth - to tell their offspring who their mothers are.

Detailing their findings in the journal Marine Mammal Science, the researchers said that each animal had a unique whistle, a fact that allows such "signatures" to be used for identification.

The researchers also highlight the fact that bottlenose dolphins are highly social, and that calves encounter many adult females in their first weeks, and may potentially mistake them for their mothers.

"The most obvious explanation for the increase in maternal signature whistle production is the need for the mother to be in contact with her calf," the BBC quoted zoologist Dr. Deborah Fripp from Dallas Zoo as saying.

"However, the decrease in signature whistle production of (dolphin) mother Lotty after three weeks does not fit this idea, especially as calves actually wander further from their mothers as they get older," she added.

She believes that whistling likely enables a process called imprinting, whereby the calf learns to recognise its mother.

"Bottlenose dolphins can swim at birth and are highly social. In other species, these traits are associated with imprinting. A calf can easily get separated from its mother and find itself among many other dolphins," she said.

Dr. Fripp said that imprinting might also help prevent females from stealing newborns from other mothers, a behaviour that had previously been observed in bottlenose dolphins (Tursiops truncatus).

"Theft incidents almost always occur in the first day of the calf's life. Perhaps this is because after a calf has imprinted on its mother, such theft is more difficult," she said.

Dr. Fripp also points out that though dolphins can whistle at birth, they are not born with their unique signature whistle.

"Dolphin mothers do not teach their babies how to whistle, so the increase in whistle production at birth is not for this," she said.

"Calves' whistles are almost never similar to their mothers'. Interestingly, female calf whistles are more similar to those animals in their environment which they are not interacting with than to those of animals they know," she added.

She, however, insisted that further research was needed to confirm her team's findings.